The neuronal endoplasmic reticulum: its cytochemistry and contribution to the endomembrane system. II. Axons and terminals.

The morphology and cytochemistry of the endoplasmic reticulum (ER) in axons and terminals of a number of different types of neurons in brains from mice were investigated ultrastructurally. The neurohypophysis received particular attention because the morphology and enzyme cytochemical activities of many of the preterminal swellings of hypothalamo-neurohypophysial axons are altered by chronic salt-stress. Membrane contrast and enzyme cytochemical staining techniques were employed to characterize the axonal reticulum and to determine if organelles representing the lysosomal system in the axon and the tubular profiles participating in the anterograde axonal transport of native horseradish peroxidase (HRP) are associated with the ER. Potential enzyme cytochemical markers for the axonal ER included glucose-6-phosphatase (G6Pase), thiamine pyrophosphatase, nucleoside diphosphatase, and acid hydroxylase activities. The anterograde transport of HRP was analyzed in undamaged hypothalamo-neurohypophysial neurons and in facial and hypoglossal motoneurons of mice receiving the protein in the lateral cerebral ventricle. The ER pervaded the axon and appeared as parallel, 20-40-nm-wide tubules interconnected by oblique anastomoses. Membrane thickness of the axonal reticulum measured 60-100 A, which is similar to that of the perikaryal ER. Enzyme cytochemical activities associated with the ER or lysosomes were not conspicuous in axons and terminals under normal conditions but became prominent in some axons and preterminal swellings manifesting an autophagic appearance within neurohypophyses from salt-stressed mice. Only G6Pase activity was a marker for the ER in these axons and preterminals. Many ER profiles in non-incubated sections and in G6Pase cytochemical preparations of salt-stressed neurohypophyses were wrapped around or interspersed among secretory granules, multilamellar bodies, and vacuoles that may represent forms of lysosomes involved in autophagy and crinophagy. Acid hydrolase activities were localized within the vacuoles as well as within 80-130-nm-wide, blunt-ended tubules in pituitary stalk axons; similar reactive tubules were confluent with large secondary lysosomes in neurosecretory cell bodies and may be derived from these lysosomes. Morphologically identical tubules transporting HRP in the anterograde direction were observed only in the salt-stressed hypothalamo-neurohypophysial neuron. The HRP-positive tubules very likely are affiliated with the lysosomal system.